Surface acoustic wave devices, i.e., surface acoustic wave filters or surface acoustic wave resonators are now increasingly used as an alternative to dielectric filters for RF-band filters used with mobile communications equipment such as portable telephones and cordless telephones. One reason for this is that a surface acoustic wave device, especially a surface acoustic wave filter is smaller in size than a dielectric filter. Another reason is that the surface acoustic wave filter is superior in electrical performance to the dielectric filter, if they are of the same size.
A surface acoustic wave device is made up of, at least, a piezoelectric substrate, a comb form of metal film electrode pattern formed on the surface of the piezoelectric substrate, and a package for housing both the piezoelectric substrate and the electrode pattern therein. Lithium niobate, lithium tantalate, quartz, etc. are used for the piezoelectric substrate. Lithium niobate, and lithium tantalate having a large electromechanical coupling coefficient are used especially for RF-band filters. Aluminum, etc. are used for the electrode pattern.
FIG. 13 illustrates a general process sequence of steps of fabricating a prior art surface acoustic wave device. At a step (b), a metal film 51 for an electrode material is first formed as by vapor deposition or sputtering on a piezoelectric substrate 50 pre-washed at a step (a). Following this, a photoresist is coated on the metal film 51 as by spin coating. Then, the photoresist is exposed to light according to a desired pattern using an exposure system, and developed to obtain a photoresist pattern 52, as depicted at a step (c). Thereafter, the metal film is etched, either wet or dry, at a step (c) into a desired electrode pattern 53. The photoresist used for pattern formation is removed at a step (e) using a stripping solution or by means of an ashing process. Thus, the pre-process called a photo-process finishes. After this, the piezoelectric substrate with the electrode pattern formed thereon is diced at a step (f) into individual chips. Then, each chip is fixed at a step (g) to a package using an bonding agent, after which bonding wires are interconnected at a step (h). Finally, a lid is welded at a step (i) to the package for ensuring airtightness, followed by performance inspection at a step (j). Thus, the so-called post-process comes to an end.
A problem with the surface acoustic wave device when used at an RF-band of about 1 GHz is that the lifetime becomes short because the electrode finger width of the comb electrode and the space between electrode fingers become as fine as about 1 .mu.m. A key determinant of the lifetime of the surface acoustic wave device is the power-durability of the electrode film. At the beginning, aluminum or Al was used for the reasons of its small specific gravity, and its low electric resistance. However, a problem with using aluminum for the electrode film is that the degradation of the electrode film becomes more pronounced as the applied frequency becomes higher. When the surface acoustic wave device is in operation, repetitive stress proportional to frequency is applied on the electrode film on the piezoelectric substrate. The repetitive stress applied on the electrode film gives rise to migration of aluminum atoms, which in turn causes electrode film defects such as hillocks, and voids, resulting in some considerable degradation of the performance of the surface acoustic wave device. The degradation of the electrode film becomes more pronounced as the applied frequency becomes higher and the applied power becomes larger. In view of design consideration, the higher the frequency, the thinner the electrode film and the narrower the electrode width should be. Because of these and other factors, the electrode film is more likely to have defects as the applied frequency become higher. In other words, the power-durability of the surface acoustic wave device becomes low.
As an approach to reducing or preventing the degradation of the electrode film due to the migration of aluminum atoms, J. I. Latham et al have disclosed the use of an aluminum-copper (Al--Cu) alloy obtained by adding to aluminum a trace amount of a different type metal such as copper (Cu) (Thin Solid Films, 64, pp. 9-15, 1979), and showed that by use of such an aluminum alloy it is possible to prevent occurrence of hillocks or voids on the electrode film and so improve the power-durability of a surface acoustic wave device.
Other examples of improving the power-durability of electrodes by using aluminum alloys obtained by adding to aluminum trace amounts of different types of metals are disclosed in many publications inclusive of JP-B 7-107967 (Al--Ti alloy), U.S. Pat. No. 2,555,072 (Al--Ge alloy), and JP-A's 64-80113 (Al--Cu--Mg alloy) and 1-128607 (Al--Zn alloy). In any case, a trace amount of different metal is added to aluminum, so that the migration of aluminum atoms is inhibited, thereby preventing degradation of the electrode. However, the addition of a different metal to aluminum is not preferable for the reason that an inevitable increase in the electric resistance of the electrode film result in an increased loss in the surface acoustic wave device.
Incidentally, it is considered that the rate of diffusion of aluminum atoms is faster at grain boundaries than in crystal grains; that is, grain boundary diffusion occurs preferentially. It is thus believed that the migration of aluminum atoms due to repetitive stress applied on a surface acoustic wave device occurs predominantly at grain boundaries, as pointed out so far in the art.
FIG. 10 is a scanning electron microscope photograph showing how an aluminum electrode film is degraded due to repetitive stress applied on a surface acoustic wave device. The fact that voids are found at grain boundaries of the aluminum electrode film supports that the migration of aluminum atoms occurs predominately at grain boundaries.
Accordingly, if grain boundaries are removed out of an aluminum electrode film or if grain boundaries are substantially reduced, that is, if an aluminum electrode film close to a single crystal is achieved, great power-durability improvements are then expectable. As known in the art, one factor of the electric resistance of a electrode film is a scattering of electrons due to grain boundaries. In this regard, too, removal of grain boundaries is preferable because, in the absence of them, there is an electric resistance decrease and, hence, the loss in a surface acoustic wave device decreases.
Application of a substantial single-crystal form of material to an electrode film for a surface acoustic wave device has already been disclosed in JP-A 55-49014. The publication alleges that by use of an electrode material that is substantially a single crystal, it is possible to enhance the performance of a surface acoustic wave device, whatever material is used to make up the device. The publication shows that molecular beam epitaxy is preferred to obtain such an electrode film. However, the publication provides no disclosure of what is used for the substrate material, and how film-forming conditions are determined depending on the electrode material used. That is, what is disclosed therein is nothing else than general consideration to the effect that an improvement in the performance of a surface acoustic wave device is expectable by use of a single-crystal electrode film. Thus, the publication provides no illustrative disclosure of to what degree the Q value, and the aging properties are improved. To add to this, there are several problems in fabricating surface acoustic wave devices at low costs by molecular beam epitaxy. For instance, costly equipment is needed, and the film-forming rate is slow.
One example of applying a single-crystal aluminum film or an aluminum film oriented in a certain crystal alignment direction to an electrode film for a surface acoustic wave device is disclosed in U.S. Pat. No. 2,545,983. Here, a rotated Y-cut quartz substrate in the 25.degree. to 39.degree. rotated Y-cut range is used as a piezoelectric substrate to obtain a (311) oriented film by vapor deposition at a high rate (a deposition rate of 40 .ANG./sec.) and a low temperature (a substrate temperature of 80.degree. C.). The patent publication states that the film is an epitaxially grown film close to a single-crystal film. In the case of low-temperature vapor deposition, it appears that a problem arises in connection with close contact of the aluminum electrode film with the underlying quartz substrate (film adhesive strength). In this regard, the patent publication suggests that an extremely thin Ti or Cr film is located at the interface between the quartz substrate and the aluminum film to such an extent that the orientation capability of the aluminum film is not inhibited. For RF-band filters where the power-durability of electrode films are a matter of great concern as already mentioned, lithium niobate and lithium tantalate are often used as piezoelectric substrates, because of the magnitude of their electromechanical coupling coefficient. However, the patent publication discloses nothing about the use of lithium niobate and lithium tantalate; it refers merely to the (311) orientation of an aluminum film when quartz is used as a piezoelectric substrate.
JP-A 5-90268 discloses a technique for providing a buffer metal layer on a piezoelectric substrate and forming a thin film form of aluminum or aluminum alloy thereon, and indicates that this structure allows the aluminum or aluminum alloy to show strong (111) orientation. So far, it has been known that the (111) oriented film of aluminum shows strong migration resistance. Consequently, the power-durability of a surface acoustic wave device should be improved by the (111) oriented electrode film. The example given therein teaches that the (111) oriented Al alloy film is obtained by using quartz as the piezoelectric substrate and forming a Ti buffer layer and an Al-0.5 wt % Cu alloy on the substrate in the described order, using a dual ion beam sputtering system. The publication teaches that the degree of the (111) orientation of the Al alloy was found from the strength of X-ray diffraction from the (111) plane, the full-width at half maximum of a rocking curve, etc. Insofar as the data about the widths of X-ray diffraction peaks and the full-widths at half maximum of rocking curves, given in the publication, are concerned, however, it appears that the obtained aluminum alloy is a polycrystalline film oriented strongly in the (111) direction.
The above publication shows an example where lithium niobate useful for making up an RF-band filter is used as the piezoelectric substrate. In this example, vanadium (v), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), yttrium (Y), chromium (Cr), zinc (Zn), Fe.sub.20 Ni.sub.80 alloy, Ti.sub.50 V.sub.50 alloy, and Co.sub.90 Ni.sub.10 are used as a buffer metal, and 300 .ANG. aluminum is deposited on the buffer metal. The example teaches that the above metal and alloy species except zinc contribute to a decrease in specific resistance and an improvement in orientation capability. However, the obtained film is actually a polycrystalline film with the (111) plane oriented, because the full-widths at half maximum of rocking curves shown therein have a minimum value of 2.1 degrees. A. Kamijo and T. Mitsuzuka disclose similar results in an article, J. Appl. Phys. 77(8), pp. 3799-3804, 1995. From this article, it is found that when films are formed at an extremely low deposition rate by an ion beam sputtering process, for instance, an aluminum film is formed at 0.2 nm/sec. and a buffer metal film is formed at 0.04 to 0.2 nm/sec., an aluminum film strongly oriented in the (111) direction is obtained. However, this aluminum film is a polycrystalline film in the intra-planar direction thereof, and so the resulting electrode film is a polycrystalline film rather than a heteroepitaxial film.
JP-A 5-199062 shows that an aluminum single-crystal film is used as a electrode film for a surface acoustic wave device, and discloses means for forming this aluminum single-crystal film. Here ST-or LST-cut quartz is used as a piezoelectric substrate. The publication alleges that by allowing the surface of this substrate to have an archipelagic structure wherein minute, hemispherical islands are almost uniformly present, it is possible to obtain an aluminum single-crystal film by a vapor deposition or sputtering process. The publication then teaches that a conventional etching process may be used as the technique for processing the surface of the substrate to impart the archipelagic structure thereto. The publication also teaches that a substrate material other than quartz, for instance, lithium niobate useful for making up an RF-band filter, too, is effective for use with this technique for obtaining a single-crystal aluminum film. In this regard, however, the publication discloses nothing about any illustrative example and data.
JP-A 6-132777 discloses another prior art regarding an aluminum single-crystal film. The publication teaches that when a film is formed on an extremely thin and clean crystal plane at an extremely low deposition rate, an aluminum single-crystal film is obtained, and shows some examples wherein an aluminum single-crystal film may be formed on an LST-cut quartz substrate by a vacuum vapor deposition process, an aluminum single-crystal film may be formed on a 128.degree. Y-cut lithium niobate substrate by a vacuum vapor deposition process, and an aluminum single-crystal film may be formed on a 112.degree. X-cut lithium tantalate substrate by a vacuum vapor deposition process. Incidentally, the publication teaches that if the surface of the substrate is contaminated, no single-crystal film is then obtained. As the deposition rate becomes slow, there is generally an increase in the probability of impurities, i.e., contaminant atoms arriving at, and being captured by, the surface of the substrate. It is thus expected that too slow a deposition rate makes it difficult to obtain a single-crystal film. Unless the degree of vacuum of an atmosphere during deposition and the deposition rate are placed under precise control, it is then impossible to obtain a single-crystal film with high reproducibility. Thus, some problems arise in connection with mass production. To obtain a single-crystal film, it is important to reduce the kinetic energy of aluminum atoms arriving at the surface of the substrate. To reduce the energy of the aluminum atoms, it is then required to achieve proper control of the temperature of evaporation of an evaporation source and the temperature at which the substrate to be provided with a film is heated, rather than the deposition rate depending on the makeup of a deposition system. However, the above publication is silent about these considerations.
As explained with reference to the prior publications, the power-durability of the aluminum electrode film used in the surface acoustic wave device becomes a matter of great concern. As the means for solving this problem, the following three methods have so far been proposed: one method where the electrode film is formed of an alloy, another method where the electrode film is formed of a film oriented in the (111) direction, and yet another method where the electrode film is formed of a single-crystal film. However, the alloy electrode film suffers from an increase in the loss in the surface acoustic wave device, which has an inevitable relation to an electric resistance increase. On the other hand, the electrode film formed of the film oriented in the (111) direction is improved in terms of power-durability due to the (111) orientation. However, it is impossible to achieve outstanding prevention of migration of aluminum atoms at grain boundaries, because too many grain boundaries are still present in the electrode film. In contrast to the above two electrode films, the single-crystal electrode film is free from migration of aluminum atoms at grain boundaries because there are no grain boundaries, and can have decreased electric resistance. Thus, this single-crystal film may be best suited for use as a power-durable electrode material. However, the single-crystal film forming techniques disclosed in the above referred-to publications have several grave problems. For instance, the deposition system used costs much, the deposition rate is too slow for mass fabrication of surface acoustic wave devices, and the substrate used is limited to quartz. Especially in the case of lithium niobate best suited for making up an RF-band filter where the power-durability of an electrode film is a matter of great concern, in particular a 64.degree. rotated Y-cut lithium niobate piezoelectric substrate, how an aluminum electrode film is converted into a single-crystal film, and how a single-crystal aluminum electrode film is fabricated remains still unclarified.
When it is difficult to form a single-crystal aluminum electrode film, on the other hand, an aluminum, polycrystalline electrode film showing high (111) orientation is useful in view of power-durability. As already mentioned, it is reported that a (111) polycrystalline film showing high orientation is obtainable by forming a buffer metal on a piezoelectric substrate and forming an aluminum electrode film thereon. When 36.degree. rotated Y-cut lithium tantalate useful for making up an RF-band filter is used as the piezoelectric substrate to achieve high (111) orientation capability, what type of buffer film should be used, and at what thickness and under what conditions the buffer film should be formed remains still unclarified.